PROJECT ABSTRACT Kawasaki disease (KD) is an acute febrile illness/systemic vasculitis of unknown etiology that predominantly afflicts young children, causes coronary artery abnormalities and aneurysms (CAA), and could potentially result in long-term cardiovascular sequelae and even death. KD vasculitis is the leading cause of acquired heart disease among children in the US. CAA develop in 25% of untreated children with KD, leading to ischemic heart disease and myocardial infarction. While Intravenous immunoglobulin (IVIG) treatment lowers the risk of CAA to 5%, up to 25% of KD patients are IVIG-resistant and have a greater risk for CAA. Therefore, discovery of more effective treatments to prevent the cardiovascular complications of KD vasculitis is a high priority in pediatric and cardiovascular research. The intestinal microbiome is an integral part of our physiology and intestinal dysbiosis influences the development of a number of immunological and non-immunological diseases, including cardiovascular diseases. In preliminary studies, we discovered striking new evidence that intestinal microbiota, gut permeability, and dysregulated mucosal immune responses play a key role in the development of coronary arteritis and aneurysm formation in KD using a well-established KD vasculitis mouse model. Based on our preliminary data, we hypothesize that intestinal dysbiosis and increased gut permeability concomitantly occur during KD and play a crucial role in modulating immune responses significantly contributing to the cardiovascular lesions associated with KD. These events will result in commensal microbiota translocation and/or bacterial/fungal PAMPs as well as increased gut permeability of metabolites, and secretory IgA into blood circulation and may play an important role in modulating and fine-tuning systemic and local immune responses helping drive the immunopathology and fuel the development of KD lesions. Deciphering the mechanisms by which the intestinal commensal micro and mycobiome and increased gut permeability affect the development of cardiovascular lesions of KD could provide a novel therapeutic target for intervention. To test this hypothesis, we propose to determine how compositional alterations of the intestinal commensals influence murine KD vasculitis pathology (Aim 1). We will investigate the role of increased intestinal permeability and determine if its prevention has therapeutic value during murine KD vasculitis. (Aim 2). We will characterize the role of secretory IgA leaking from the gut in promoting the development of cardiovascular lesions in KD vasculitis model (Aim 3). Clinical data suggest that children with KD frequently have a leaky gut and more than 80% receive microbiome altering antibiotics in the week prior to KD diagnosis. Therefore, this proposal has a very high translational potential given that specific manipulation of the commensal microbiota is a research area with high therapeutic promises. Understanding the role and the molecular mechanism by which gut microbiome and gut permeability contribute to the cardiovascular complications of KD may lead novel therapeutic and preventive approaches.